The present invention relates to a single-layer type electrophotosensitive material, which is used in image forming apparatuses such as electrophotographic copying machine, facsimile and laser beam printer. More particularly, the present invention relates to a single-layer type electrophotosensitive material, which is less likely to cause wear of a photosensitive layer due to a blade cleaning means that recovers a residual toner and is therefore superior in wear resistance, and also does not cause defects such as image fog and black belt due to reduction of a surface potential and lowering of the sensitivity even when exposed to ozone generated in a charging, transferring or separating means in the image forming apparatus and is therefore superior in ozone resistance.
In an image forming apparatus based on a Carlson process, an image has conventionally been formed by uniformly charging the surface of an electrophotosensitive material, exposing the surface of the electrophotosensitive material to light to form an electrostatic latent image, developing the electrostatic latent image to form a toner image, transferring the toner image onto a transfer material (e.g. paper) and fixing the toner image. After transferring, the electrophotosensitive material is repeatedly used for a long period after cleaning of the residual toner and destaticization.
It is, therefore, necessary that electrophotosensitive material is not only superior in initial electric characteristics such as initial charge characteristics and initial sensitivity characteristics, but also in repeated electric characteristics such as charge stability and sensitivity stability so as to endure periodic duty for a long term.
The electrophotosensitive material is classified roughly into an inorganic photosensitive material using an inorganic material such as selenium in a photosensitive layer, and an organic photosensitive material using an organic material in a photosensitive layer. Among these photosensitive materials, the organic photosensitive material has widely been studied because it is easily produced as compared with the inorganic photosensitive material and has a wide range of choice of photosensitive materials such as electric charge transferring material, electric charge generating material and binder resin as well as high functional design freedom.
The organic photosensitive material is classified roughly into a so-called multi-layer type photosensitive material having a structure of an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material, which are mutually laminated, and a single-layer type photosensitive material wherein an electric charge generating material and an electric charge transferring material are dispersed in the same photosensitive layer. Among these organic photosensitive materials, it is a multi-layer type photosensitive material, which has a monopoly position in the wide market. The multi-layer type photosensitive material is exclusively a negative charging type photosensitive material comprising a conductive substrate, and an electric charge generating layer and an electric charge transferring layer formed in order on the conductive substrate.
The single-layer type photosensitive material has become of major interest recently because of its advantages described below. That is, the single-layer type photosensitive material is superior in productivity because of its simple layer construction and can inhibit the occurrence of layer defects of the photosensitive layer, and can also improve optical characteristics because of less interface between layers. Furthermore, one photosensitive material can be used as both of positive and negative charge type photosensitive materials by using, as the electric charge transferring material, an electron transferring material and a hole transferring material in combination.
In the image forming apparatus based on the Carlson process, to which an organic photosensitive material is mounted, in case a charging means, a transferring means or a separating means is composed of a charger system, the photosensitive material is exposed to ozone generated by corona discharge. Since ozone is a gas having a very strong oxidizing action, the strong oxidizing action is exerted on the photosensitive material. The strong oxidizing action is exerted on constituent substances of the photosensitive layer (e.g. electric charge generating material and electric charge transferring material) in a binder resin, which forms a photosensitive layer of the organic photosensitive material, thereby causing problems such as lowering of the charge capability of the photosensitive material and lowering of the sensitivity, and thus the repeated electric characteristics become worse.
When using the organic photosensitive material having lowered charge capability as it is, the surface potential is reduced, thereby to cause image defects such as image fog, black belt, densification of gray images and thickening of image letters.
In an image forming apparatus based on a Carlson process, an image is formed by uniformly charging the surface of an electrophotosensitive material, exposing the surface of the electrophotosensitive material to light to form an electrostatic latent image, developing the electrostatic latent image to form a toner image, transferring the toner image onto a transfer material (e.g. paper) and fixing the toner image. After transferring, the electrophotosensitive material is repeatedly used for a long period after cleaning of the residual toner and destaticization.
It is, therefore, necessary that electrophotosensitive material is not only superior in initial electric characteristics such as initial charge characteristics and initial sensitivity characteristics, but also in repeated electric characteristics such as charge stability and sensitivity stability so as to endure periodic duty for a long term.
In order to achieve excellent repeated electric characteristics, it is essential to use an organic photosensitive material, which is less likely to cause wear of a photosensitive layer due to a blade cleaning means that recovers a residual toner and is therefore superior in xe2x80x9cwear resistancexe2x80x9d, and also does not cause reduction of a surface potential and lowering of the sensitivity even when exposed to ozone generated in a charging, transferring or separating means in the image forming apparatus and is therefore superior in xe2x80x9cozone resistancexe2x80x9d.
Influence of Wear Resistance
The developed toner is transferred to a transfer material such as paper in the transfer process. However, the toner is not transferred completely (100%) and is partially remained on the photosensitive material. If the residual toner is not removed, it is made impossible to obtain a high-quality image, which is free from contamination in the repeated processes. Therefore, it is required to clean the residual toner. In the cleaning process, a fur brush, a magnetic brush or a blade is typically used. In view of the cleaning accuracy and rationalization of apparatus construction, it is general to select a blade cleaning wherein cleaning is performed by contacting a blade-shaped resin plate directly with a photosensitive material.
Although the blade cleaning has high accuracy, it increases a mechanical load on the photosensitive material, thereby causing problems such as increase in wear amount of the photosensitive layer, reduction in surface potential, lowering of the sensitivity, and thus the repeated electric characteristics become worse.
Influence of Ozone Resistance
In case a charging means, a transferring means or a separating means is composed of a charger system, the photosensitive material is exposed to ozone generated by corona discharge. Since ozone is a gas having a very strong oxidizing action, the strong oxidizing action is exerted on the photosensitive material. The strong oxidizing action is exerted on constituent substances of the photosensitive layer (e.g. electric charge generating material and electric charge transferring material) in a binder resin, which forms a photosensitive layer of the organic photosensitive material, thereby causing problems such as lowering of the charge capability of the photosensitive material and lowering of the sensitivity, and thus the repeated electric characteristics become worse.  less than influence of wear resistance greater than 
When using the organic photosensitive material having poor wear resistance, poor ozone resistance and lowered charge capability as it is, the surface potential is reduced, thereby to cause image defects such as image fog, black belt, densification of gray images and thickening of image letters.
Unlike the multi-layer type photosensitive material, the electric charge generating material also exists on the outermost surface of the photosensitive layer as far as the single-layer type photosensitive material is provided with an overcoat layer. To the contrary, in case of a negative charging multi-layer type photosensitive material, the electric charge generating material is protected with the electric charge transferring material. Therefore, the electric charge generating material is likely to be exposed to ozone generated in the image forming apparatus and, therefore, an influence of ozone is likely to be exerted on the single-layer type photosensitive material.
The wear resistance and the ozone resistance have a close relation. In case of designing a so-called xe2x80x9clong-lifexe2x80x9d photosensitive material, which does not cause image defects such as fog even when copying a lot of sheets, a technique of using a resin having an improved wear resistance or various lubricants is disclosed. Although the wear resistance of the photosensitive layer is improved by the above technique, it becomes difficult to skive the surface portion of the photosensitive layer whose charge capability was lowered by exposing to ozone generated in the image forming apparatus, and thus a xe2x80x9clong-lifexe2x80x9d photosensitive material can not be obtained.
To improve the ozone resistance, Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) Nos. 6-161134, 6-222589, 6-222588 and 6-214413 suggest an image forming apparatus using an organic photosensitive material wherein an ozone concentration in the image forming apparatus is not more than 0.5 ppm and an oxygen transmission rate (oxygen permeability coefficient) per unit thickness of the electric charge generating layer is not more than 1.0xc3x9710xe2x88x927 cc/cm2xc2x7Sxc2x7cmHg.
Although various studied have been made to control the ozone concentration in the image forming apparatus to 0.5 ppm or less through recent consideration to the environment, the ozone concentration is within a range from 0.5 to 5.0 ppm at present in the vicinity of a charger in an image forming apparatus wherein a charging, transferring or separating means is composed of a charger system, or a large-sized high-speed image forming apparatus, and thus image defects such as image fog occur sometimes.
As described above, the multi-layer type photosensitive material is exclusively a negative charging type photosensitive material and the electric charge generating layer is laminated on the electric charge generating layer formed on the conductive substrate. Therefore, the electric charge generating material in the electric charge generating layer is not exposed directly to ozone generated in the image forming apparatus. To the contrary, in case of the single-layer type photosensitive material, since the electric charge generating material also exists on the outermost surface layer and is likely to be exposed directly to ozone, an influence of ozone is likely to be drastically exerted as compared with the multi-layer type photosensitive material.
Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 7-152162 describes that an electrophotosensitive material wherein an oxygen transmission rate of a photosensitive layer is not more than 2.0xc3x97103 cm3/m2xc2x724 hrxc2x7atom is superior in ozone resistance. Although there is some description about a bisphenol A type polycarbonate resin (i.e. poly(4,4,xe2x80x2-dimethylmethane-diphenylcarbonate) in the Examples of the above Japanese Published Unexamined Patent Application, it has been found that sufficient ozone resistance can not be obtained when using the resin in the single-layer type photosensitive material.
Thus, an object of the present invention is to provide a single-layer type electrophotosensitive material, which is less likely to cause wear of a photosensitive layer even when used in an image forming apparatus wherein a cleaning means that recovers a residual toner is a blade cleaning means, and also does not cause defects such as image fog and black belt due to lowering of the charge capability and sensitivity even when used in an image forming apparatus wherein charging, transferring or separating means is composed of a charger system, or an image forming apparatus having an ozone concentration of 5.0 ppm at most such as large-sized high-speed image forming apparatus and is therefore superior in ozone resistance.
To attain the objects described above, the present inventors have intensively studied and found such a novel fact that a single-layer type electrophotosensitive material comprising a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein an oxygen transmission rate of the binder resin is not more than 35 ccxc2x7mm/m2xc2x7dayxc2x7atom does not cause defects such as image fog and black belt due to lowering of the charge stability and lowering of sensitivity even when used in an image forming apparatus wherein charging, transferring or separating means is composed of a charger system, or an image forming apparatus having an ozone concentration of 5.0 ppm at most such as large-sized high-speed image forming apparatus and is therefore superior in ozone resistance.
That is, the single-layer type electrophotosensitive material of the present invention has a feature that it comprises a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein an oxygen transmission rate of the binder resin is not more than 35 ccxc2x7mm/m2xc2x7dayxc2x7atom.
According to the present invention, it is estimated that the surface of the photosensitive layer formed by using the binder resin, through which oxygen molecules do not easily permeate, i.e. the binder resin having a small oxygen transmission rate, includes a small amount of voids larger than that of oxygen molecules and ozone molecules (larger than oxygen molecules) generated in the image forming apparatus do not easily diffuse into the photosensitive layer from the surface of the photosensitive layer, and thus the electric charge generating material or electric charge transferring material in the photosensitive layer is less likely to be exposed to ozone molecules, thereby making it possible to obtain a single-layer type electrophotosensitive material having excellent ozone resistance.
The present inventors have intensively studied furthermore based on the knowledge described above and also found such a novel fact that a single-layer type electrophotosensitive material comprising a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein the electric charge transferring material contains an electron transferring material and a hole transferring material in which a mobility at an electric field strength of 5xc3x97105 V/cm of the hole transferring material is not less than 5xc3x9710xe2x88x926 cm2/V/sec and a solid content of the electric charge transferring material is not less than 30% by weight and not more than 50% by weight based on the whole solid content, and an oxygen transmission rate of the binder resin is not more than 35 cc mm/m2xc2x7dayxc2x7atom is less likely to cause wear of the photosensitive layer even when used in an image forming apparatus wherein a cleaning means that recovers the residual toner is a blade cleaning means and is therefore superior in wear resistance, and does not cause defects such as image fog and black belt due to lowering of the charge capability and lowering of the sensitivity even when used in an image forming apparatus wherein charging, transferring or separating means is composed of a charger system, or an image forming apparatus having an ozone concentration of 5.0 ppm at most such as large-sized high-speed image forming apparatus and is therefore superior in ozone resistance.
That is, the present invention includes a single-layer type electrophotosensitive material of the present invention comprising a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein the electric charge transferring material contains an electron transferring material and a hole transferring material in which a mobility at an electric field strength of 5xc3x97105 V/cm of the hole transferring material is not less than 5xc3x9710xe2x88x926 cm2/V/sec and a solid content of the electric charge transferring material is not less than 30% by weight and not more than 50% by weight based on the whole solid content, and an oxygen transmission rate of the binder resin is not more than 35 ccxc2x7mm/m2xc2x7dayxc2x7atom.
According to the present invention, it is estimated that the surface of the photosensitive layer formed by using the binder resin, through which oxygen molecules do not easily permeate, i.e. the binder resin having a small oxygen transmission rate (35 ccxc2x7mm/m2xc2x7dayxc2x7atom or less), includes a small amount of voids larger than that of oxygen molecules and ozone molecules (larger than oxygen molecules) generated in the image forming apparatus do not easily diffuse into the photosensitive layer from the surface of the photosensitive layer, and thus the electric charge generating material or electric charge transferring material in the photosensitive layer is less likely to be exposed to ozone molecules, thereby making it possible to obtain a single-layer type electrophotosensitive material having excellent ozone resistance.
As the solid content of the electric charge transferring material, which serves nearly as a plasticizer in the binder resin, becomes smaller (not less than 30% by weight and not more than 50% by weight), the wear resistance of the photosensitive layer is further improved. To the contrary, the transferring efficiency of charges is lowered and the sensitivity becomes worse. However, a single-layer type photosensitive material having sufficient sensitivity can be obtained by using the hole transferring material having large mobility (mobility at the field strength of 5xc3x9710xe2x88x925 V/cm is not less than 5xc3x9710xe2x88x926 cm2/V/sec) even if the solid content of the electric charge transferring material is small.
As described above, a single-layer type photosensitive material, which is superior in wear resistance and ozone resistance, can be obtained by using the hole transferring material having large mobility, thereby to reduce the solid content of the electric charge transferring material, and using the binder resin having a small oxygen transmission rate.
As described above, the single-layer type electrophotosensitive material of the present invention has a feature that it comprises a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein an oxygen transmission rate of the binder resin is not more than 35 ccxc2x7mm/m2xc2x7dayxc2x7atom has good ozone resistance, and does not cause defects such as image black belt due to lowering of the charge capability even when used in an image forming apparatus having an ozone concentration of 5.0 ppm at most, such as an image forming apparatus wherein charging, transferring or separating means is composed of a charger system.
Furthermore, the single-layer type electrophotosensitive material of the present invention comprising a conductive substrate and a photosensitive layer made of a binder resin containing at least an electric charge generating material and an electric charge transferring material, which is formed on the conductive substrate, wherein the electric charge transferring material contains an electron transferring material and a hole transferring material in which a mobility at an electric field strength of 5xc3x97105 V/cm of the hole transferring material is not less than 5xc3x9710xe2x88x926 cm2/V/sec and a solid content of the electric charge transferring material is not less than 30% by weight and not more than 50% by weight based on the whole solid content, and an oxygen transmission rate of the binder resin is not more than 35 ccxc2x7mm/m2xc2x7dayxc2x7atom is superior in ozone resistance, sensitivity and wear resistance.